The present invention concerns a thermohydraulic mixing grid for nuclear fuel assembly, this grid being designed to improve the mixing of the streams of coolant circulating in the assembly and to homogenize the temperatures, and being intended to be inserted between grids which are the structural components of the assembly.
The invention finds a particularly important application in fuel assemblies for pressurized water reactors comprising a support structure having two nozzles connected by thimble guides and having fuel rod positioning grids distributed along the assembly, each positioning grid having at least two sets of intersecting straps and an outer surround delimiting the cells crossed some by the thimble guides and others by the rods, the straps being provided with means for supporting the rods at the nodes of a regular lattice and being fixed to at least some of the thimble guides, some grids at least (those placed downstream in the flow direction) being provided with mixing vanes for the coolant streams.
At least one of the positioning grids also supports the fuel rods. To do so, it is normally fitted with springs, cut out of the straps or added on to the straps, designed to force the rods against dimples stamped in the straps forming the sides of the cell.
The other positioning grids only perform a function of centering the rods at the nodes of the lattice. To do so, they comprise rod support dimples on each side of each cell crossed by a rod.
To increase heat exchange between the rods and the coolant, and to homogenize the temperatures at each level, it has already been proposed to insert an additional mixing grid, in one or more of the intervals between two positioning grids.
In particular, mixing grids have been proposed with low internal straps, fitted with vanes, having a surround. The surround must then consist of outer straps which are thick and/or higher than the inner straps, increasing the neutron absorption and head losses. The surround must be sufficiently strong to avoid being deformed when two grids of two adjacent fuel assemblies collide or in the case of earthquake.
An attempt was made to circumvent the problem (EP-A-Q 261 544) by using additional grids devoid of surrounds, consisting of flat straps, fixed to the thimble guides and delimiting cells with dimensions such that the rods pass through them with a clearance.
Such intermediate grids present drawbacks. They do not participate in positioning the rods. The peripheral channels do not have means for mixing the liquid streams, and are cooled less than the internal channels, incurring the risk of overheating.